c - 如何用C重写文件的全部内容

Question

我有用于 ajax 源的文本文件。浏览器每 1 秒发送一次 ajax 请求以从此文件中读取实际数据。我也有写在 C 上的守护进程，它将实际数据写入该文件。看下面的代码:

static void writeToFile_withLock(const char * file_path, const char * str)
{
    struct flock fl = {F_WRLCK, SEEK_SET,   0,      0,     0 };
    int fd;
    const char * begin = str;
    const char * const end = begin + strlen(str);

    fl.l_pid = getpid();

    if ((fd = open(file_path, O_CREAT | O_WRONLY)) == -1) {
        perror("open");
        exit(1);
    }
    printf("Trying to get lock...\n");
    if (fcntl(fd, F_SETLKW, &fl) == -1) {
        perror("fcntl");
        exit(1);
    }
    printf("got lock\n");

    printf("Try to write %s\n", str); 
    while (begin < end)
    {
        size_t remaining = end - begin;
        ssize_t res = write(fd, begin, remaining);
        if (res >= 0)
        {
            begin += res;
            continue; // Let's send the remaining part of this message
        }
        if (EINTR == errno)
        {
            continue; // It's just a signal, try again
        }
        // It's a real error
        perror("Write to file");
        break;
    }

    fl.l_type = F_UNLCK;  /* set to unlock same region */

    if (fcntl(fd, F_SETLK, &fl) == -1) {
        perror("fcntl");
        exit(1);
    }

    printf("Unlocked.\n");

    close(fd);

}

问题:如果以前的数据 > 新数据，那么旧的几个符号保留在文件的末尾。

我如何重写完整的文件内容？

提前致谢。

Answer 1

将 O_TRUNC 添加到 open() 调用中...

O_TRUNC

If the file already exists and is a regular file and the open mode allows writing (i.e., is O_RDWR or O_WRONLY) it will be truncated to length 0. If the file is a FIFO or terminal device file, the O_TRUNC flag is ignored. Otherwise the effect of O_TRUNC is unspecified.